Udn has been proposed as a promising technique for 5G to further enhance the network capacity [1]}, [2]}. The key idea of UDN is to increase the number of bss and deploy small cells [3]} in the cell-centric cellular network. However, as the increase of the cell density, the inter-cell interference grows larger and larger. The cooperation theory has determined that [4]}, the upper limit of the network capacity will be bounded by the inter-cell interference as long as the network-centric network is used. In other words, inter-cell interference becomes the bottleneck for the network capacity improvement of UDN. This problem is inherent to the cell-centric network paradigm, and cannot be efficiently solved [5]}.
